Negative density dependence in the mortality and growth of tropical tree seedlings is strong, and primarily caused by fungal pathogens

Natural enemies have been implicated as agents of negative density dependence (NDD) in tropical forests, but their relative contributions to NDD, and thus to the maintenance of diversity, are largely unknown. We monitored the rates of survival and relative growth rates on seedlings for 10 years in tropical moist forest in Manu National Park, Peru. We then experimentally manipulated the plots to exclude fungal pathogens, insects, small mammals and large mammals for an additional 31 months to assess the influence of these natural enemies on density-dependent interactions among tropical seedlings. Fungal pathogens made the most important contribution to NDD. The application of fungicide led to lower mortality rates, faster growth rates and decreased species diversity. Other taxa of natural enemies had at most minor effects on seedling performance. Synthesis. We conclude that fungal pathogens are the strongest contributors to the widely observed NDD that occurs among seedlings. Moreover, the presence of fungal pathogens augments the species diversity of seedlings, indicating their critical contribution to the maintenance of species coexistence and the structure of tropical tree communities.